It's a no-brainer for me, because:
- Less time spent reading paper maps = more time spent riding.
- Planning trips is waaaay easier.
- I'm never lost, even if I have no idea where I am.
- Provides the accurate log of my travels. I can superimpose those logs on Google Maps or Google Earth and share them with other people.
- I can import other people's GPS logs and follow in their footsteps.
- Provides geo-tagging for all the pictures I take on the road. I'll never forget where I've taken a picture again. Check this photo location for an example:
http://picasaweb.google.com/lh/photo/r05NQmCPRNM0eXeUyDgmMA?feat=directlink- Gives me cool statistics of my travels, such as altitude profiles, time spent moving, average speeds, etc. Example here:
http://picasaweb.google.com/lh/photo/vBfkP5efM88g5mnO2zDXPA?feat=directlink- With some practice and care GPS can be used as a heads up display when the visibility is impaired (blind corner, fog, etc.).
There's more, but these are the major points. I have omitted the obvious reasons for using a GPS, like quickly finding the nearest Outback Stakehouse in Vegas to have dinner before your flight back home.
I have a Garmin E-Trex Vista HcX and use it for hiking, driving, motorcycle and bicycle riding. It's compact, light and portable. Runs on two AA rechargeable batteries for more than 24 hours (generic USB power supplies work for car/bike). It has a micro SD slot, and one 2GB card can hold the entire US/Canada detailed map and months worth of track logs. No voice prompts or Bluetooth, but it's a good trade-off for price and portability.